In the United States, people tend to think of measles as a rare childhood illness, easily preventable through routine vaccination. But in much of the world, large numbers of children still die from this highly contagious disease. In areas where many infants are born HIV-positive, the risk is even worse. But a new study suggests that repeated measles vaccinations may help.

Measles killed more than 450,000 people in 2004, according to the U.S. Centers for Disease Control. More than half of those deaths were in sub-Saharan Africa, a region also hard hit by HIV and AIDS.

William Moss is an epidemiologist at the Johns Hopkins Bloomberg School of Public Health. He and his colleagues have been studying the effectiveness of measles vaccines given to children in Lusaka, Zambia. In their study, 441 children were vaccinated, including 66 who were infected with HIV.

"The children were given measles vaccine at nine months of age," Moss explains, "and then asked to return either one or three months after vaccination for evaluation and for a blood test." The test measured the concentration of protective antibodies in the children's blood following vaccination.

Previous studies had suggested that measles vaccination might not be effective in HIV-infected children. Surprisingly, Moss found that within six months of vaccination, almost as many HIV-infected children developed protective levels of measles antibodies, as did those who were HIV-negative.

He and his colleagues then had the children return for follow-up tests at two and three years of age. "As we followed these children over time, the protective [measles] antibody levels in the HIV-infected children declined."

As would be expected, the mortality rate of the HIV-infected group was very high. In fact, over the course of the study, 28 of the HIV-positive children died.

But Moss found that of those who survived, only about half retained protective antibody levels, compared to about 90 percent of the uninfected children.

In 2003, Zambia conducted a highly successful measles prevention campaign, vaccinating an estimated 97 percent of children between six months and 14 years of age. For Moss, the campaign provided an additional opportunity, by allowing him to analyze the response to re-vaccination of a small number of the HIV-infected children.

Moss found that about 90 percent of those children had protective antibody levels, following the second vaccination. These results suggest that "in the short term, re-vaccination was able to protect these children," says Moss.

The World Health Organization recommends that all children be vaccinated twice to protect them from measles infection. Moss's research indicates that repeated vaccinations may be especially critical in countries with high rates of HIV-infection.

Access to HIV medications in Zambia has dramatically increased since Moss and his colleagues began their research. They plan to start a new study in October, to examine the effects of measles vaccination in children who are receiving antiretroviral therapy.

The results of their current study will be published in the August 1 in The Journal of Infectious Diseases. Their paper is also available online.